I. Field of the Invention
This invention relates to control of extrusion blow molding machines. In particular this invention relates to control of mechanisms determining wall thickness of an extruded parison produced by an accumulator type extrusion blow molding machine. II. Description of the Prior Art
Extrusion blow molding machines are known (from, for example U.S. Pat. No. 4,424,178) wherein an accumulator is provided from which a predetermined quantity of plasticized material is extruded to produce a hollow tubular parison to be blow molded to form a finished article. Further, it is known from this patent to provide means for varying wall thickness as the plasticized material is extruded from the accumulator. In particular, programmably controlled tooling means are known to vary the annular gap through which plasticized material is to be extruded. In conjunction with such known gap control it is known to vary the speed at which material is forced through said gap. Heretofore control of speed variation has been effected in response to progress of purge of the accumulator independently of control of the tooling means. Hence synchronization of gap variation and variation of extrusion(purge) speed has been imprecise tending to impair optimization of program parameters and particularly prohibiting maximization of purge speed hence prohibiting reduction of cycle time.